1. Field of the Invention
This invention relates to a method of and apparatus for severing a sheet of refractory material and, more particularly, to scoring bulb edges of a glass ribbon for subsequent removal.
2. Discussion of the Technical Problems
As taught in U.S. Pat. No. 3,998,616 in the manufacture of a glass ribbon by the float method, a continuous ribbon of molten glass is controllably cooled as it moves through a forming chamber on a pool of molten metal. The ribbon exits the chamber as a dimensionally stable continuous ribbon. During the forming of the ribbon, the edges of the molten glass are selectively acted on when a ribbon thickness other than equilibrium thickness, e.g., about 0.27 inch (0.686 centimeter) is desired. For example, for ribbon thicknesses less than equilibrium thickness, as the molten glass moves through the forming chamber the edges are contacted by attenuating machines to move the edges away from one another. The glass ribbon, after exiting the forming chamber, moves through an annealing lehr and is thereafter cut into sections, usually called lehr ends.
The edges of the annealed ribbon usually referred to as bulb edges, are removed because they are areas of high stress and of unacceptable optical quality. In one bulb edge removal technique the ribbon is scored in areas of low stress, e.g., about 10 inches (25.4 centimeters) in from the ribbon edge to attain a smooth cut edge. A smooth cut edge is preferred for ease of storing and for safe handling of the glass. Although the above technique gives a relatively smooth cut edge, the ribbon width reduction of 20 inches (0.508 meter) is uneconomical.
In another technique, two scores are imposed in each bulb edge with the first score spaced about 3 inches (7.62 centimeters) from the adjacent ribbon edge and a second score spaced about 2.5 inches (0.98 centimeter) from the first score, i.e., about 5.5 inches (2.17 centimeters) from the adjacent ribbon edge. The score closest to the ribbon edge is opened first and usually the resultant cut edge is jagged because of the high stress level of the scored edges. Initial removal of the bulb edge at the first score reduces the stress level such that opening the second score yields a relatively smooth cut edge. A technique employing two scores to remove the bulb edge of a glass ribbon or a lehr end is taught in U.S. Pat. No. 4,285,451 which teachings are hereby incorporated by reference. Although the above technique provides a relatively smooth cut edge while maximizing the ribbon width, a limitation is the equipment and time required for imposing two scores in each of the bulb edges and sequentially opening each score.
In still another technique, identical scores are imposed in each opposed surface of the ribbon at each of the bulb edges. If the two scores at the bulb edge are perfectly aligned with one another, i.e., the score in the top surface of the glass ribbon is perfectly aligned with the score in the bottom surface of the ribbon, the resultant cut edge is smooth. However, if they are not aligned, the two scores do not interact with one another and the scores randomly open, i.e., each score opens as if the other was not present. The result is an uneven and unpredictable cut edge.
As can now be appreciated, it would be advantageous to provide a bulb edge removal technique that does not have the drawbacks of the presently available techniques.